1. Technical Field of the Invention
The present invention relates generally to a solenoid-operated valve device designed to enhance the response speed of a solenoid actuator to move a valve, and a fuel injection system using such a valve device.
2. Background Art
Solenoid-operated valve devices are in wide-spread use which produce magnetic attraction to move a valve to open or close a fluid path. The solenoid-operated valve devices are quipped with a solenoid valve which includes an electromagnet working to attract the valve in a given direction when energized. The solenoid valve is also equipped with an elastic member such as a spring which urges the valve in a direction reverse to that in which the valve is magnetically attracted. When the electromagnet is deenergized, the valve is held in an initial position by the elastic member. This type of solenoid valve has the drawback in that when a cycle in which the valve is open or closed is shortened, the magnetism created in the coil will result in a decrease in response speed at which the valve is closed.
In order to alleviate the above program, Japanese Patent First Publication No. 2000-265920 discloses a solenoid-operated valve device which uses a valve-opening coil and a valve-closing coil to enhance the response speed at which the valve is opened and closed. Japanese Patent First Publication No. 2001-59462 also teaches controlling the valve-opening coil and the valve-closing coil to facilitate the disappearance of magnetism remaining in one of the coils.
In recent years, fuel injection systems for diesel engines such as common rail injection systems have been required to increase the pressure at which fuel is injected into the engine and enhance the accuracy in controlling a small quantity of fuel to be injected into the engine. To meet such requirements, some of the fuel injection systems employ fuel injectors which are designed to use high-pressure of fuel in assisting in moving a valve of the fuel injector or a hydraulic mechanism working to amplify a stroke of a piezoelectric device hydraulically to improve the response speed of the fuel injectors. The fuel injectors equipped with such a piezoelectric device are designed not to drain the fuel statically for spraying the fuel, which is made to use the negative pressure of fuel in moving the valve in conventional fuel injectors. This results in a difficulty in brining the pressure of fuel in the common rail into agreement with a target value, for example, when an accelerator pedal is released suddenly, so that the target value of the pressure in the common rail is decreased greatly, which may cause the pressure of fuel which is higher than the target value to be injected into the engine. When it is required to elevate the pressure of fuel to be sprayed from the fuel injectors, it may also result in an overshoot thereof, thus requiring the need for increasing the pressure resistance of component parts of the fuel injectors and resulting in the complexity of the structure thereof. When an actual pressure of the fuel in the common rail is much higher than the target value, ensuring the desired accuracy in controlling the quantity of fuel to be sprayed from the fuel injectors requires draining the fuel from the common rail quickly, thus resulting in the need for actuating a pressure control solenoid valve at high speeds which drains the fuel from the common rail. Particularly, when it is required to the time the fuel injectors should be opened accurately, it is essential to improve the response speed of the pressure control solenoid valve.
The solenoid-operated devices, as taught in the above two publications, are useful for improvement the response speed of the valve, but however, use the two coils and thus require driver circuit one for each of the coils, thus resulting in an increase in component parts and complexity of the structure thereof. Additionally, when the voltage to be applied to the solenoid-operated valve is increased in order to enhance the speed of valve action thereof, it requires, for example, large-capacity capacitors, thus resulting in an increase in component parts and complexity of the structure of the solenoid-operated valve devices.